There are three standard milling procedures for separating gold from its ores: flotation, amalgamation and cyanidation.
In the flotation process, particles of finely ground ore are separated from each other according to the abilities of the different metals in the ore to attach themselves to an oily froth. The oily froth is created by a frothing agent which causes water to foam, a collecting agent which forms a film on the gold so that it will stick to air bubbles which rise to the top of the water, and various inorganic chemicals which prevent other metals from filming. Air is forced into the liquid mixture, whereby gold particles are carried to the top in air bubbles and are skimmed off in a froth at the surface.
Amalgamation is a process utilizing a principle that gold tends to form an amalgam (alloy) with mercury. In this process, the gold ore is finely ground and mixed with water to form a pulp which is passed over copper plates coated with mercury. The gold is attracted to and captured by the mercury to form a mercury-gold amalgam. The amalgam is scraped from the copper plates and then heated to boil off the mercury from the gold.
In the cyanidation process, ground ore is placed in a tank containing a weak solution of cyanide. The gold in the solution is precipitated by its contact with metallic zinc. The precipitate is melted and usually cast into gold bars. Cyanidation is dangerous because of the poisonous nature of cyanide vapor.